In recent years, cars have been equipped with an idling stop function which stops an engine when the car comes to a stop, or an electric power steering wheel which takes the load off the engine. These two items are environmental friendly and improve fuel efficiency. A hybrid system or an electric turbo system, which positively complements the drive of engine, will be used widely in the market. On top of that, car manufacturers have proposed various ideas about a car brake such as an electrical hydraulic brake that will replace a conventional mechanical hydraulic brake.
As discussed above, the car tends to need electric power increasingly from now on; however, a battery, having conventionally powered to the car, cannot supply an instantaneous large amount of power only by itself, so that it sometimes fails to supply sufficient power. If the battery becomes abnormal, the driving system possibly fails to operate normally.
To overcome the foregoing problems, an electric storage device is proposed as an auxiliary power supply for supplying enough power even when the battery becomes abnormal. The electric storage device is disclosed in, e.g. patent document 1.
FIG. 13 shows a schematic circuit diagram of one of the foregoing conventional electric storage devices. In this circuit, each one of energy storage device cells 110 is formed of an electrically double-layered energy storage device having large capacitance, and it is employed as an electric storage element. Multiple energy storage device cells 110 are coupled together in series to form energy storage device pack 112, which is coupled with a power supply such as a battery for charging energy storage device cells 110.
Each one of energy storage device cells 110 is coupled in parallel with a load, e.g. balancing resistor 114, in order to balance the voltages across each cell 110 with each other. Between energy storage device cell 110 and resistor 114, relay switch 116 is connected. Relay switch 116 is formed of a regular-open type relay-contact 116a working as a switch section and electromagnetic coil 116b driving the switch section. Electromagnetic coil 116b is coupled in parallel between the power supply and the ground, and accessory switch 118 is coupled to the power supply. A turn-on of switch 118 thus drives every electromagnetic coil 116b, thereby turning on every relay switch 116.
The work of the foregoing electric storage device is described hereinafter. When a car is started up, accessory switch 118 is turned on with an ignition key, and then every relay switch 116 is turned on as discussed above. Energy storage device cell 110 and balancing resistor 114 are thus coupled together in parallel, so that the power supply starts charging respective energy storage device cells 110. The voltages across each one of energy storage device cells 110 are adjusted automatically to be equal by respective balancing resistors 114. This mechanism allows preventing an over-charge to energy storage device cell 110, so that a longer service life of energy storage device cell 110 can be expected.
When the car stops, accessory switch 118 is turned off, then every relay switch 116 is turned off, so that energy storage device cell 114 is separated from balancing resistor 114. As a result, each one of energy storage device cells 114 becomes independent of the wiring, and cells 114 stand storing the electric charges which have been charged just before the car stops. Energy storage device cell 110 can be thus prevented from discharging needlessly electric charges, and can be kept storing the electric charges for a long period. The foregoing work of the electric storage device allows supplying enough power to restart the engine.
The electric storage device discussed above allows preventing an overcharge to energy storage device cell 110, and thus extending its service life. It also allows reducing needless discharge from energy storage device cell 110 during the halt of the car, and supplying enough power to restart the engine. However, if energy storage device cell 110 is left for a long time with the electric charges stored during the halt of the car, the stored electric charges spontaneously discharge, so that the voltages across respective energy storage device cells 100 lower gradually. Since energy storage device cells 110 have dispersion in their characteristics, the dispersion causes the spontaneous discharge to produce another dispersion in the voltages across respective cells 110. If the car is started up in this state, relay switch 116 is turned on, thereby connecting balancing resistor 114 to energy storage device cell 110, so that the dispersed voltages across respective cells 110 are adjusted automatically to a certain value by balancing resistor 114. FIG. 14 shows the time-varying progress of voltages across cells 110. In FIG. 14, the horizontal axis represents the time, and the vertical axis represents the voltages across respective energy storage device cells 110.
As shown in FIG. 14, at time “t0” when the car is started up, if the voltages across respective cells 110 disperse within the dispersive range, energy storage device cell 110 is charged while the disperse is adjusted, so that the voltages across respective cells 110 increase. At time “t2”, the voltages across cells 110 become almost equal to each other, i.e. reach voltage V1.
Since a greater dispersive range needs a longer time t2 necessary to reach voltage V1, when the car is left halting only for a short time, the dispersive range becomes small, so that the conventional electric storage device allows reaching voltage V1 within a relatively short time. However, if the car is left halting for a long time, the disperse in spontaneous discharge of energy storage device cell 110 causes the time span (hereinafter referred to as a balancing time) for balancing the voltages with each other to be extremely longer, such as in the order of several hours. This balancing time is determined by a time constant found from the capacitance of energy storage device cell 110 and the resistance value of balancing resistor 114. In this case, the capacitance of the electric storage device to be used as the auxiliary power supply to the car is already determined, so that the resistance value of balancing resistor 114 affects the balancing time.
A greater resistance value of resistor 114 is preferable for minimizing the discharge from energy storage device cell 110 and for reducing unnecessary power consumption; however, an excessively great resistance value will prolong the balancing time, so that the resistance value is obliged to set at a certain value. This setting necessarily incurs a problem that the balancing time at the start-up of the car becomes longer depending on a dispersive range, caused by leaving the car for a long time, of the voltages across energy storage device cells 110. As a result, energy storage device cell 110 is overcharged for a longer time before the voltages are balanced, so that the service life of energy storage device cell 110 is possibly shortened.    Patent Document 1: Unexamined Japanese Patent Application Publication No. H10-201091.